System for controlling the diameter of a crystal in a crystal growing furnace

Abstract

A crystal growing furnace has lift and rotation control apparatus for both a growing crystal and a crucible containing a melt. A transducer and a servo motor are disposed above the furnace having respective shafts connected along a common horizontal axis on opposite sides of a longitudinal light tube wherein the light tube and transducer shaft can be axially positioned by the shaft of the servo motor. The light tube has a lens for focusing at times on a spot on a meniscus in the furnace and at times on the melt, a light sense detector is connected to the light tube for at times generating a servo motor control error signal to drive the servo motor incrementally to an angular position corresponding to growth in the diameter of the meniscus by tracking a spot on the meniscus. The transducer generates an output signal indicative of incremental angular movement in response to growing the crystal for governing the lift and rotation control apparatus.

What is claimed is: 
     
       1. The method of controlling the diameter of a crystal in a crystal growing furnace having lift and rotation control means for both the crystal and a crucible containing a melt comprising the steps of: (a) sensing a spot of light on a meniscus at an interface between a growing crystal and the melt by transmitting said spot through an angularly operable light tube to an electrical transducing means,   (b) generating an electrical signal corresponding to the light intensity senses,   (c) generating an error signal in response to variations in the electrical signal,   (d) driving a servo motor shaft connected to the light tube to actuate the light tube angularly in response to the error signal,   (e) driving a shaft of an angular transducer by the shaft of the servo motor,   (f) generating a transducer error signal in response to rotation of the shaft of the transducer,   (g) governing the rate of lift and rotation by the transducer error signal,   (h) a further step of electronically limiting angular operation of the light tube by the servo motor to positions between inside and outside angular limits determined by a range of crystal diameters for which the method is used as sensed by electrical signals from the angular transducer and controlled by light levels through the light tube, and   (i) a further step of causing the light tube to scan an area of the melt between the inside and outside limits in case of a momentary power interruption to relocate the meniscus for continuation of the normal tracking operation.   
     
     
       2. The method of controlling the diameter of a crystal according to claim 1 wherein the method comprises a further step of electronically limiting the angular rate of operation of the light tube, by the servo motor, by electronically deriving the angular rate from the angular position transducer.